1. Field of the Invention
The present invention relates to: a conductive member used in an image forming apparatus such as a copying machine, a laser-beam printer and a facsimile machine; a process cartridge having the conductive member; and an image forming apparatus having the process cartridge.
2. Description of Related Art
Conductive members are used as a charging member for applying a charging process to an image carrier (a photoconductor) and a transferring member for applying a transferring process to a toner on the image carrier in image forming apparatuses of a conventional type which uses an electrophotographic method, including electrophotographic copying machines, laser printers and facsimile machines. FIG. 10 is an explanatory diagram of an image forming apparatus which includes a charging member, and which uses an electrophotographic method of a conventional type
In FIG. 10, reference numeral 300 denotes an image forming apparatus using the electrophotographic method of the conventional type. The image forming apparatus 300 using the electrophotographic method of the conventional type is configured of: an image carrier 221 on which an electrostatic latent image is formed; a charging roller 212 for performing a charging process while contacting the image carrier 211; exposing means 213 using a laser beam or the like; a developing device 220 including a toner carrier (developing roller) for adhering a toner 215 to the latent image on the image carrier 211; a transferring member (transferring roller) 216 for carrying out a process for transferring the toner image on the image carrier 211 onto a recording medium 217; and a cleaning device 221 including a cleaning member (cleaning blade) 218 for cleaning the image carrier 211 after the transfer process is completed. In FIG. 10, reference numeral 219 denotes a waste toner.
Descriptions will be provided next for how the image forming apparatus 300 using the electrophotographic method of the conventional type operates basically for forming an image.
When a DC voltage is supplied from a voltage supply (not illustrated) to the charging roller 212 which is brought into contact with the image carrier 211, the surface of the image carrier 211 is evenly charged by the charging roller 212. Once an image light is irradiated on the surface of the image carrier 211 by the exposing means 213 immediately after this charge, a potential drops depending on an amount of the light in a part of the surface of the image carrier 211 on which the image light is irradiated. The mechanism through which such a charging roller 212 charges the surface of the image carrier 211 is based on discharge in a minute interstice between the charging roller 212 and the image carrier 211. The mechanism is known as Paschen's law.
The image light represents distribution of the amount of the light, which distribution indicates changes in the amount of the light depending on black and white of the image. For this reason, once such an image light is irradiated thereon, the irradiation of the image light forms distribution of potentials depending on the amount of the image light, or an electrostatic latent image, on the surface of the image carrier 211. Once a portion in the surface of the image carrier 211 in which the electrostatic latent image is formed goes through the developing roller 214, the toner 215 adheres to the surface of the image carrier 211 depending on the distribution of potentials. Thus, the electrostatic latent image is visualized as a toner image. Thereafter, the recording medium 217 is transported by a resist roller (not illustrated), and thus is superimposed on the toner image. Hence, the toner image is transferred to the recording medium 217 by the transferring roller 216. After the toner image is transferred to the recording medium 217, the recording medium 217 is separated from the image carrier 211. The recording medium 217 thus separated is transported through a transporting channel. After the image is heated and thus fixed to the recording medium by a fixing unit (not illustrated), the resultant recording medium is discharged out of the image forming apparatus. Once the image transfer is completed in this manner, a cleaning process is applied to the surface of the image carrier 211 by the cleaning blade 218 in the cleaning device 221. Subsequently, a quenching lamp (static eliminator, not illustrated) removes residual charges from the surface of the image carrier 211, and thus makes the image carrier 211 ready for the next round of the image transferring process.
Image forming apparatuses of a type using a contact charging method in which the charging roller is brought into contact with the image carrier has been known as the image forming apparatus using such a general charging method in which the foregoing charging roller is used. The image forming apparatus using the contact charging method has been disclosed in Japanese Patent Application Laid-open Publication Numbers Sho. 63-149668 and Hei. 1-267667. Nevertheless, the image forming apparatus using the contact charging the method has disadvantages as follows.    (1) A substance constituting the charging roller is easy to adhere to the image carrier.    (2) The substance constituting the charging roller oozes from the charging roller, and accordingly adheres to the surface of the image carrier. If this condition progresses, a trace of the charging roller remains on the surface of the image carrier.    (3) When a DC voltage is applied to the charging roller, the charging roller being in contact with the image carrier vibrates. This causes charging noise.    (4) Parts of the toner on the image carrier adhere to the charging roller, and this deteriorates the charging characteristic of the charging roller. In particular, after the substance constituting the charging roller oozes therefrom as described in (2), parts of the toner is easier to adhere to the charging roller.    (5) In a case where the image carrier remains out of operation for a long period of time, a permanent deformation takes place in the charging roller.
Image forming apparatuses of a type using a proximity charging method have been disclosed as techniques for solving the foregoing problems in Japanese Patent Application Laid-open Publication Numbers Hei. 3-240076 and Hei. 4-358175. In the case of the proximity charging method, the charging roller is not in contact with the image carrier. Instead, the charging roller is caused to come closer to the image carrier with a certain gap interposed between the charging roller and the image carrier. In the case of the charging devices of this type using the proximity charging method, the charging roller is placed opposite to the image carrier in a way that the distance between the charging roller and the image carrier is equal to the closest distance (5 μm to 300 μm), and a voltage is applied to the charging roller so that the image carrier is charged. Image forming apparatuses using this proximity charging method are free from the problems with image forming apparatuses using the conventional contact charging method, such as the problem of “the adherence of the substance constituting the charging roller to the image carrier” and the problem of “the permanent deformation which takes place in the charging roller in the case where the image carrier remains out of operation for a long period of time.” That is because the charging roll is not in contact with the image carrier. In addition, the image forming apparatuses using the proximity charging method are less likely to “deteriorate the charging characteristic of the charging roller due to the adherence of parts of the toner on the image carrier to the charging roller” than the image forming apparatuses using the contact charging method. That is because parts of the toner adhere to the charging roller in a smaller amount.
In the image forming apparatuses using the proximity charging method described in Japanese Patent Application Laid-open Publication Numbers Hei. 3-240076 and Hei. 4-358175, a spacer ring is provided between the two end portions of the charging roller for the purpose of maintaining a gap between the charging roller and the image carrier. Nevertheless, no arrangement is made for setting the gap accurately in these image forming apparatuses using the proximity charging method. For this reason, the charging roller and the spacer ring vary in dimensional accuracy, and the gap in between accordingly varies in clearance. This brings about a problem that charged potential is uneven and varies in the image carrier.
An image forming apparatus of a type which including tape-shaped gap maintaining means with a predetermined thickness between the charging roller and the image carrier for the purpose of solving the above-described problems has been disclosed in Japanese Patent Application Laid-open Publication No. Hei.5-107871. Nevertheless, the image forming apparatus of the type which includes the tape-shaped gap maintaining means has a problem that, if the image forming apparatus is used for a long period of time, the tape-shaped gap maintaining means wears out, and this makes it impossible for the gap between the surface of the image carrier and the surface of the charging roller to maintain a certain clearance. In addition, parts of the toner enters an interstice between the charging roller and the tape-shaped gap maintaining means, and the parts of the toner stick to the interstice due to a portion of an adhesive which has extruded from the tape-shaped gap maintaining means. This changes the thickness of the tape-shaped gap maintaining means. The changed thickness brings about a problem of making it impossible for the gap between the surface of the image carrier and the surface of the charging roller to maintain the certain clearance.
Furthermore, for the purpose of solving such a problem, Japanese Patent Application Laid-open Publication No. 2005-91818 has disclosed a charging roller including an elongate conductive supporting body constituting a shaft member, an electrical resistance adjusting layer formed on the circumferential surface of the conductive supper, and a pair of gap maintaining members provided respectively to the two ends of the electrical resistance adjusting layer in a way that the gap maintaining members are in contact with the two respective ends. In the conductive member of this type, the gap maintaining members are securely fixed to the conductive supporting body by applying an adhesive to an interstice between the conductive supporting body and each of the gap maintaining members for the purpose of enhancing the long-term reliability. However, the coefficient of linear expansion of the gap maintaining members made of a synthetic resin is largely different from the coefficient of linear expansion of the conductive supporting body made of a metal. This brings about a problem that, in a case where the charging roller is placed under a high-temperature or low-temperature condition, the conductive supporting body and the gap maintaining members are likely to be detached from each other in their interface so that the long-term reliability deteriorates slightly. In addition, the charging roller in which the adhesive is applied to the interstice between the conductive supporting body and each of the gap maintaining members has a problem that, if the charging roller is electrified for a long time, the electrification decreases the adhesive strength so that the gap maintaining members move from their initial positions, and the charging roller is easy to charge the image carrier unevenly due to the variation in the gap.
The gap maintaining members and the electrical resistance adjusting layer are made of different materials in consideration of the sticking tendency. The electrical resistance adjusting layer needs to have a tendency to cause the toner to stick to the electrical resistance adjusting layer. Accordingly, an ionic conductive agent with higher water absorption properties is used as a resistance adjuster for the electrical resistance adjusting layer. This brings about a problem that, under a high-temperature and high-humidity condition, the electrical resistance adjusting layer absorbs moisture so that the dimensions of the electrical resistance adjusting layer vary. On the other hand, the gap maintaining members need to have insulating properties and a tendency to prevent the toner from sticking to the gap maintaining members. For this reason, it is desirable that a polyolefin-based resin material be used as a resin material constituting the gap maintaining members. However, the polyolefin-based resin material is a material exhibiting less water absorption. For this reason, the amount of dimensional variation of each of the gap maintaining members is smaller than the amount of dimensional variation of the electrical resistance adjusting layer under a high-temperature and high-humidity condition. This brings about a problem of varying the gap accurately formed between the surface of the image carrier and the surface of the charging roller.
The gap accurately formed between the surface of the image carrier and the surface of the charging roller is formed on the basis of a step provided to a joint section between the electrical resistance adjusting layer and each of the gap maintaining members. The step provided to the joint section between the electrical resistance adjusting layer and each of the gap maintaining members is formed through removing processes inclusive of cutting and grinding an external portion of the electrical resistance adjusting layer and an external portion of each corresponding one of the gap maintaining members, the gap maintaining members being in pair and provided to the two respective ends of the electrical resistance adjusting layer in a way that the gap maintaining members are in contact with the two respective ends. During the removing processes inclusive of the cutting and grinding processes, burrs are easy to produce while the outside is easy to protrude, in a part of the external portion of one of the gap maintaining members at which the processes start and in the stepped portions. For this reason, when the charging roller is fitted to the image carrier, these burrs and the protrusion of the outside come between the image carrier and the charging roller. This brings about a problem of making it impossible to secure the accuracy with which the gap is maintained. Another problem occurs particularly during the removing processes inclusive of cutting and grinding the external portions of the gap maintaining members and the electrical resistance adjusting layer. When a tool cuts into workpieces, the workpieces change in shape due to the process resistance (elastically deformation). After processed, the deformed portions return to their original shape, and protrude. Yet another problem is that, due to the process resistance, chips are easy to adhere to the portion at which the processes start while burrs are easy to produce at the portion.
In addition, while the process provided to the joint section between the electrical resistance adjusting layer and each of the gap maintaining members is being processed, the tools are moved toward the center portion of the external diameters respectively of the electrical resistance adjusting layer and each of the gap maintaining members. This process imposes a heavier load on the workpieces, and accordingly brings about a problem that: burrs are easy to produce in the workpieces while chips are easy to adhere thereto. Another problem is that, while the joint section between the electrical resistance adjusting member and each of the gap maintaining members is being processed, burrs are easy to produce in the joint section while chips are easy to adhere thereto, the material of the electrical resistance adjusting member being different from the material of the gap maintaining members. Yet another problem is that the processing of the different materials affects the deterioration degree and life of each of the tools and whetstone.